Our invention relates generally to cathode-ray tubes (CRTs) and more particularly to those of the type incorporating a "meshless" electron lens system, commonly referred to as the scan expansion or deflection amplification lens system, for expanding the deflections of the electron beam in both vertical and horizontal directions. Still more particularly, our invention deals with an improved scan expansion lens system of generally boxlike, two-electrode configuration for high-speed CRTs for both oscilloscopic and information storage use, among other application.
The meshless scan expansion lens systems of various known configurations have almost superseded the more conventional fine-metal, dome-shaped mesh lens in today's high-performance CRTs. Each such meshless lens system comprises several lens elements of electrodes positioned in alignment about the tube axis of the CRT so as to encompass the trajectors of the electron beam from the deflection fields to the target.
We know some prior art meshless lens systems, examples being those disclosed in Japanese Laid Open Patent Applications Nos. 59-134531, 60-65436 and 60-23939 and U.S. Pat. No. 4,302,704 to Saito. The lens system suggested by the first mentioned reference is very complex in construction, comprising two nested cylindrical electrodes, a slot lens at the beam entrance end of the electrodes, and an aperture lens at their exit end. The other three references are alike in teaching the use of boxlike lens elements configured to provide quadrupolar lens actions. We prefer boxlike lens systems because of their simplicity of construction and ease of fabrication.
The current trend in the design of CRTs, however, is the reduction of their size, particularly of their axial dimension. This objective demands, of course, the advent of a meshless lens system of smaller axial length.